418 
GEOLOGY OF THE FIRST DISTRICT. 
6. Potsdam Sandstone. 
This rock underlies the Calciferous sandrock; and this latter is intermediate in composi¬ 
tion, as well as in age, to the Potsdam sandstone and Mohawk limestone. It seems to have 
been deposited during the period when the cause producing the Potsdam sandstone was 
diminishing in its effects, and that of the limestone had commenced, and was increasing. 
The Potsdam sandstone is a hard siliceous sandstone, white, red, grey, yellowish, and 
frequently striped. It is well developed at Whitehall, where it has a thickness of one hundred 
and fifty to two hundred feet. It extends up the valley of Wood creek in a southerly direction 
by Comstock’s landing, one mile east of Fort-Ann; and so on farther south, diminishing in 
thickness, and becoming interlaminated with finer grained strata of grits, slates and shales. 
Some of the strata of this rock are covered with the most beautifully characterized ripple- 
marks, as perfect as if just formed on the sand of a sea-beach, while the rock is of the most 
indurated kind of sandstone. Fine examples of these may be seen in the flagging stones in 
the sidewalks of the village of Fort-Ann. Many places in the vicinity, and between that vil¬ 
lage and Comstock’s landing might be referred to, where they may be seen in the rocks in 
place. Fucoidal impressions were also seen in some of the strata of this rock. 
The Potsdam sandstone is used but little, and that little for flagging and building. It can 
be quarried in fine blocks twelve to sixteen inches thick, with plain surfaces on the bed and 
top, but it is a hard stone to dress. It is one of the most durable kinds of rock. 
Boulders and pebbles of the Potsdam sandstone, and the granular quartz of the Bennington 
range, (and which I believe to be the same rock more changed as a metamorphic rock,) forni 
a large share of the mass of the gravel and pebble beds of the plains along the Hudson, and 
more particularly in the high pebble-bottomed valleys of the highlands of the Hudson. Some 
of these pebbles are so compact and hard, as to pass for coarse red and striped jasper. 
The Potsdam sandstone was seen in several places ; and in some, its lower part in parti¬ 
cular, is a metamorphic rock having more or less the aspect of gneiss, except that mica is 
absent. In another locality, half a mile west of Putnam ferry, where it overlies granitic 
rocks, it is in an intermediate state, having the general aspect of a primary rock, but still 
shows its rounded gravel and sand. The usual dip is to the eastward, at variable angles from 
five to twenty degrees; but in the Hudson valley, where it is exposed in numerous localities, 
upturned with other rocks, it dips at high angles to the eastward. The Potsdam sandstone, 
somewhat altered in texture by metamorphic agency, may be seen at the south end of Mount 
Stessing, underlying the limestone of the Calciferous group. Many localities of this rock 
might be mentioned, but the rock can be best studied between Whitehall and Fort-Ann, and 
in the valley of the west branch of Wood creek in Washington county, and in Galway and 
Greenfield in Saratoga county. 
